Parallels Workstation sells for $50 but you can request a licensed activation key for free using this Lunarpages offer. Key in your email address and they’ll immediately send you the download links for the trial version and serial number to convert the trial into a full retail version.

In case the above offer expires by the time you read this, you can always download the free Virtual PC (Microsoft) or Sun’s VirtualBox which is available for Mac as well.